I have a 2001 sunsetter, and it already has 2 alpine type s speakers on the tower. I recently bought two more type s speakers and an audio shop guy had told me that I could "bridge" the speakers. Is this safe, and if so how do I do it?

Depends on your amp. (if you have an amp)If your amp is 2ohm stable then wire the + on the new speaker to the + on the old speaker and - to the - on each side. That would be a 2ohm load to your amp. I see speakers wired to the deck the same way. Should be fine either way.